DC/AC converters produce an AC voltage with the grid frequency from a DC voltage in order to deliver power to the utility grid. The quality of the current injected to the grid by a DC/AC converter is of great importance according to the grid interconnection regulatory standards (e.g. IEEE1547 standard). To this end, a filter is required to filter out the harmonics of the voltage produced by the DC/AC converter and to thereby inject a high quality current to the utility grid.
There are three general types of filter used to attenuate the harmonic contents of the current injected to the utility grid. The commonly used filters are: purely inductive L-filters, LC-filters, and the LCL-filters. Recently, LCL-filters have received a lot of attention as they can provide superior filtering compared to the other filter types. This is due to the additional poles introduced by the LCL-filter structure. However, these two additional complex conjugate poles introduce a relatively low frequency resonance into the control system, thereby making control of the converter very challenging. The control system should be able to provide enough damping for the resonance produced by these complex conjugate poles.
To address the above issue, it is known to use a Proportional Resonant (PR) controller in conjunction with a linear state-feedback controller in order to damp the resonance of the LCL-filter and to thereby control the output current of the DC/AC converter. The linear state-feedback controller provides damping for the closed-loop control system (ensures stability) and the PR-controller provides current reference tracking. The arrangement shown in FIG. 1 shows this approach.
According to FIG. 1, the control system requires two extra measurements (iinv, vc) in order to implement the linear state-feedback. The extra sensors required to implement the closed-loop control system contribute to the overall cost of the DC/AC converter. Also, they require extra conditioning circuitry and respective ADCs (Analogue-to-Digital Converters) for digital implementation. Therefore, the extra sensors significantly increase the cost and complexity of the control system.
Based on the above, there is therefore a need for another approach to the above issue. A control scheme that does not require extra sensors and ADC circuitry would meet such a need and can reduce the cost and complexity of DC/AC converters.